1. Field of the Invention
The present invention relates to a stepping motor drive device, in particular to a technology for driving a stepping motor with low noise and low vibration.
2. Description of the Related Art
Late years, stepping motors are used in image capturing electronics devices, such as a DSC (Digital Still Camera) and a DVC (Digital Video Camera), as optical system actuators for adjusting aperture, focus, zoom and so on.
Stepping motors, especially those used for image capturing electronics devices, are required to operate with low noise and low vibration. This is because sounds generated by a stepping motor are caught by a built-in microphone of the electronics device and recorded as noise, while vibrations generated by a stepping motor cause image blurring and result in degradation of the recorded image.
In response to such a demand, a drive technology for stepping motor operations with low noise and low vibration has been disclosed, for instance, by Japanese Laid-Open Patent Application No. H06-343295.
FIG. 28 is a drive device disclosed by this publication. The following description will focus only on components necessary for explaining the principle of the drive device.
In FIG. 28, reference numeral 20 denotes a stepping motor that is a controlled object, reference numeral 45 is a rotor, and reference numerals 19a and 19b are a first coil and a second coil, respectively.
An up-down counter 43a counts up or down a clock signal CLKP according to an up-down signal DA, and provides a 4-bit signal DA1–DA4, which indicates the counted value, to a D/A converter 44a. 
The D/A converter 44a outputs a voltage signal VCA corresponding to the 4-bit signal DA1–DA4. The voltage signal VCA is a staircase waveform that increases and decreases stepwise. The rate of change in the voltage signal VCA can be altered depending on the pulse frequency of clock signal CLKP and the step height of voltage signal VCA, and accelerates as the pulse frequency of clock signal CLKP becomes higher., or the step height of voltage signal VCA becomes larger. In addition, the voltage signal VCA can be maintained constant by stopping the clock signal CLKP.
In a voltage drive circuit 39a, the voltage signal VCA is amplified by a non-inverting power amplifier 41a, and also by an inverting power amplifier 42a. Thus, the first coil 19a which is connected between output terminals of the power amplifiers 41a and 42a is driven by the application of a voltage.
The components similar to ones for the first coil 19a above are provided for the second coil 19b, and the same operations are performed in order to drive the second coil 19b by applying the staircase voltage.
According to this configuration, the applied voltage increases stepwise at the start of the period for current supply to the coil, while the applied voltage decreases stepwise when the supply is stopped. Herewith, vibration and noise induced by abrupt torque fluctuations at the start and stop of current supply are reduced.
However, as to the drive device of the related art above, the attenuation of the coil current at the time when the staircase signal is decreasing is slow, and therefore the coil current poorly follows the staircase signal. The same is true if for instance a staircase signal approximating a sine wave (hereinafter, referred to simply as ‘an approximate sinusoidal staircase signal’) is used, with a view to reducing vibration and noise of the stepping motor. The coil current does not exactly follow the approximate sinusoidal staircase signal. As a result, this presents the first problem that an intended reduction in vibration and noise cannot be obtained.
This is a serious problem particularly when the stepping motor is applied to image capturing electronics devices, and thus there is still a demand for further reduction in noise and vibration of the stepping motor operations.
In addition, there is a second problem that the drive device of the above related art requires a considerable amount of power since the coil current is continuously supplied.